2015|Online electrical engineering & Electrical Interview Questions

Thursday, August 27, 2015


What is the difference between power and watts, and kva and kvar and kW ? 

Being an electrical student you should be aware of the power and the units for it. power is the amount of work done divided by time. 1 watt is 1 joule per second.

So how to define electric power ?  

Electric power is nothing but rate at which the charge moves across a potential.when there is movement of charge , it produces current.
Power (P)=  V*(Q/t) = V*I
V= Voltage or potential(Volts)
I = Current (Ampere)
Q = Charge ( Coulomb)
t = Time ( Seconds)

In case of electric power , there is not only real power(P) but there is also reactive power(Q) due to inductive nature of load. All domestic load and industrial loads are inductive in nature. Real power is the does the useful work whereas reactive power doesn't do any useful work at the load , instead it wastes energy.

Example: when a fan is running , the useful work done is the rotation of it which is done by the real power consumption and the coil of the motor consumes reactive power. The unit of the reactive power is VAR .ie) Volt-Ampere Reactive .Usually it is mentioned in kVAR .

Apparent power(S) is the magnitude of real power and reactive power. When the load is reactive either inductive or capacitve, there exists a phase angle (φ) between the voltage and current. Power factor is defined as the ratio of Real power to apparent power. The unit of apparent power is VA ie) Volt-Ampere .Usually it is represented as KVA .

S= P+jQ
S^2 = P^2 +Q^2

Power factor = cos φ . Power factor has not unit and it ranges from 0 to 1. Since the industrial loads are bulk loads, they use to have capacitor banks to improve the power factor , so that the power wastage is less and electric bills too.

Hope now it will be easy to understand the difference between KVA and KW and also the difference between KVA and KVAR